Getting pounded so deep that the testicles enter the anus
"Dude, Greg was deeper than deep, he was so deep that he gave her the Buck Double"
During sex, when a man and a woman share a double sided dildo, thus simultaneously penetrating the anus of both individuals during vaginal intercourse.
I had a Buck Double last night. It wasn't at all what I expected, and I can't believe they actually advertise it in public.